A variety of normal and malignant lymphoid cell lines were found to produce inhibitors of mitogen and antigen induced blastogenesis. Inhibitors were produced by protein synthesis under specific conditions of cell proliferation and crowding. Inhibitors produced by normal and malignant human and murine lymphoid cell lines will be studied for mechanism of production, mechanism of action, species and tissue specificity, and relative reactivity against normal and malignant human and murine lymphoid tissues in vitro. Appropriate inhibitors will be purified and characterized by a variety of physicochemical and immunochemical methods. After purification those active against murine cells will be tested for immunosuppressive and antineoplastic properties in the mouse in vivo. These studies will elucidate mechanisms of contact inhibition, of cell interaction, of control of cellular proliferation, particularly in lymphoid tissue, and of control mechanisms of the immune response. They may also produce biological reagents with eventual usefulness in clinical immunosuppression and antineoplastic therapy.